If the 'manipulator' stops, that's what you get.
The real problem for the SNB now is that in order to create the peg in the first place and hold the CNF at an artificially depressed value (compared to market conditions) the SNB had to purchase significant EUR holdings in order to fight market demand.
Now the SNB has to unwind these EUR holdings, but to do so will exacerbate the original condition they were trying to hold back, i.e. they now have to unwind and sell those EUR holdings which will cause further upward pressure on the CNF.
This is how all of what I'd call "artificial pegs" end. Not only do they fail, but after they fail all the manipulation now goes in the opposite direction. Same is true for other current currency manipulations (i.e. China).
The only method a peg can work is if there is a "fixed" quantity controlled on both sides of the peg and the market can trade back and forth across that freely. A good example was the US gold standard that lasted for a period of time. In it the US fixed an amount of gold per dollar, to do so it had to maintain the gold:dollar reserve ratio by only issuing dollars in relation to the gold it held. i.e. to release a new dollar required acquiring the same amount of gold.
Inevitably governments fail at this (such as FDR's massive default), but it is only because governments always end up releasing more dollars than they have reserves for because it is free and easy, but market reality always catches up. The other way pegs fail is when an entity does not control the supply on both sides.
A peg where both sides are fixed in quantity or only go up/down in fixed relation to each other can work.